A Soviet factory that produces the main rocket motors for the U.S.S.R.'s newest long-range nuclear missile suffered a major explosion last week and is shut down, the Defense Department said yesterday.

In a statement released in reaction to a report broadcast by NBC News, the Pentagon said the explosion occurred May 12 and "destroyed several buildings at a Soviet propellant plant in Pavlograd."

"Apparently, this will delay Soviet solid-propellant missile programs," the Defense Department added.

The Pavlograd plant, located about 500 miles southwest of Moscow in the Ukraine, is the only facility the Soviets have for manufacturing the main rocket motors for the new SS24 intercontinental ballistic missile (ICBM), administration officials said.

That missile is a large 10-warhead weapon that can be launched from underground silos or rail cars. The Soviets began deploying the SS24 -- on rail cars -- last year. Fewer than a dozen are thought to be operational.

The SS24 is one of two mobile ICBMs in the Soviet arsenal. The other is the SS25, which is carried on a truck launcher. The United States has yet to deploy any mobile long-range nuclear-tipped missiles, although the Reagan administration is pressing Congress to develop a rail launcher for the MX missile.

The explosion at the Soviet plant occurred two weeks after a blast destroyed one of two U.S. plants that make a critical ingredient for the solid-rocket fuel that powers the space shuttle and various American military rockets.

It also occurred just two weeks before President Reagan and Soviet leader Mikhail Gorbachev open a summit meeting in Moscow that is expected to focus on prospects for an arms accord that would reduce the number of long-range missiles in each nation's arsenal.

Administration officials said it would probably take the Soviets at least six months to resume production.

The sources said they had no idea what had caused the blast at the Pavlograd plant, nor whether there had been casualties.

U.S. spy satellites detected the explosion on the night of May 12, a source said. The blast did not involve any nuclear warheads, another source said, "but it sure tore that plant up."

The sources said they did not know whether the plant, in assembling the main rocket motor for the SS24, was handling the oxidizer ammonium perchlorate.

One of two U.S. plants that manufacture that chemical, owned by the Pacific Engineering & Production Co., was flattened May 4 by explosions that killed two people and injured 300 others.

Pacific Engineering lost nearly 3 million pounds of the oxidizer that was to be delivered to the Air Force in June for use in the Titan rocket program, said Keith Rooker, a company spokesman.